Long‐term intensity decrease in the 8‐ to 25‐MeV proton fluxes at low L values

Abstract

A five year continuous observation, 1963–1968, of the 8‐ to 25‐MeV proton population, at L < 2.0, had shown a monotonic decrease in this population. We have observed the same proton population from 1970 to 1976, using experiments flown on several USAF satellites (72‐1, S3‐2, S3‐3). These data, together with published data from the DIAL satellite, show that the decreases in the proton fluxes first observed from 1963 to 1968 have continued unabated, at least until August 1976, and with the same original mean lives. The proton flux at L=1.35 decayed over the 13‐year period (1963–1976) with a mean life τ of 5.7±0.5 years. At L=1.90, τ was 4.55±0.16 years. However, the proton flux at L=1.20, which had first been reported as constant, started decreasing ∼ 1970–1976 with τ=3.07±0.25 years. Possible explanations for this phenomenon can be divided into the two categories of natural and artificial effects. We reviewed these different effects and conclude that most likely we are seeing the decay of the high energy protons redistributed by the ‘Starfish’ high‐altitude nuclear explosion.